Thecodonts have long been recognized as the stock from which all other archosaurs (such as dinosaurs and crocodilians) were derived. They arose in the Late Permian and diversified throughout the Triassic Period. They can be distinguished from the primitive archo-sauromorphs by the presence of an opening in the skull in front of the eye, the antorbital fenestra.
Thecodonts are a diverse assemblage and their interrelationships are not fully known and agreed upon by experts, but, as a whole, they show the initiation of archosaurian tendencies. I have recognized three distinct lineages of thecodonts based on the configuration of ankle structure: Proterosuchia, Pseudosuchia, and Ornithosuchia (for the latter see Chapter 2). In these groups the nature of articulation of two large ankle bones, the astragalus and calcaneum, is the basis for classification. In Proterosuchia, these bones are firmly articulated without any movement between them. This is the most primitive lineage, and includes various early forms such as Archosaurus, Proterosuchus, and Erythrosuchus. Proterosuchians were powerful, big-headed predators, with sprawling limbs similar to the crocodilians.
In the pseudosuchian lineage, there is a peg and socket joint between the two ankle bones allowing rotational movement between them. The peg is on the astragalus, and the socket is on the calcaneum. This pattern of ankle joint is also recognized in modern crocodiles and is referred to as the "crocodile-normal" pattern. Phytosaurs, aetosaurs, and rauisuchians are included within the Pseudosuchia. Phytosaurs were long-snouted aquatic animals similar to modern gharials in India. Aetosaurs were heavily armored herbivores with erect gait (all four limbs directly under the body) and superficially resembled the ankylosaur dinosaur. Rauisuchians were the largest terrestrial carnivores during the Middle and Late Triassic with lengths of up to 20 feet, and a developed erect posture. Some of the rauisuchians, such as Postosuchus, were highly specialized bipeds, with a swivel wrist joint as found in very advanced theropod dinosaurs and birds, and looked like a scaled-down version of Tyrannosaurus. Postosuchus was the lord of the predators in the American Southwest. The pseudosuchian lineage eventually gave rise to crocodilians, probably in the Late Triassic.
TRIASSIC WARFARE: COMPETITION AND EXTINCTION
Darwin considered the struggle for existence in a wide sense, including the competition of organisms for possession of common places in nature, as well as their destruction of one another. When food is abundant and the food chain is stabilized, different animals may coexist without much interference However, when food is in short supply, animals with similar ecologic requirements will compete with each other. Consequently, one competitor will inevitably exploit the resource more efficiently and will increase numerically at the expense of the other. In general, the progressive types prevailed, and the archaic forms disappeared under the pressure of competition. The history of Triassic vertebrates depicts three such episodes of interspecific competition and subsequent replacement of one species by another.
The replacement of therapsids by thecodonts is the first evidence of competitive exclusion in the Triassic. In the Permian, therapsids were the dominant predators in the ecosystem. In the Early and Middle Triassic, the therapsid monopoly on the large predator role was broken in a series of steps by the adaptive radiation of thecodonts. Thecodonts became larger, and developed erect posture. Their skull was fairly large, equipped with the sharp teeth of a formidable carnivore. Bipedality developed in several thecodont lines, which permitted them to move about more quickly than the sprawling therapsids. The forelimbs, being freed from locomotory function, were used for capturing and killing prey. Cynodonts could not compete with thecodonts for food and were soon replaced by them.
The second wave of extinction may be related to over-predation of many species. In the early part of the Late Triassic (Carnian stage), thecodonts and dinosaurs coexisted in tropical forest environments in many parts of the world. The lush vegetation must have supported abundant food for herbivores, as well as cover and camouflage for a variety of prey and predators. A large number of prey species was available to thecodonts and theropods (the carnivorous dinosaurs), as evident from the fossil record. Dicynodonts, rhynchosaurs, protoro-saurs, and trilophosaurs are the most common groups in the adequately known fauna. They were slow and defenseless and were accessible to predators. I have recovered rhynchosaur and protorosaur remains in the stomach contents of phytosaurs (early theropods). On the other hand, lizards and sphenodontids were small, and could hide or escape from predators by climbing or digging. The aetosaurs were heavily armored for protection and could only be subdued by large predators such as rauisuchians with their stabbing teeth. Thecodonts were more robust and larger than the contemporary dinosaurs and at this era were still able to compete successfully with dinosaurs wherever they came into extensive contact with them.
At the end of the Carnian, a major crisis developed in terrestrial ecosystems. Most of the accessible prey species such as rhynchosaurs, large trilophosaurs, and dicynodonts became extinct in response to over-predation. Both thecodonts and theropods contributed heavily to the decline and extinction of these groups. The food chain became unbalanced, and prey-switching became inevitable. Competition for food may have become more intense among large predators.
The third wave of extinction can be seen at the end of the Triassic, when thecodonts were replaced by dinosaurs. The crisis was compounded at this time, as arid climates became more widespread, and the environment became more open savanna. Due to scarcity of vegetation and decline of prey populations, the struggle for existence became severe, and the balance of competitive advantage between thecodonts and dinosaurs shifted. In an open-country environment, great speed and endurance were advantageous for survival. Although some thecodonts approached erect posture, they walked flat-footed and were relatively slow in locomotion. In contrast, early dinosaurs were fully erect, gracile bipeds, and walked on their toes like birds. The early dinosaurs could run faster with longer strides than the contemporary thecodonts. The refined locomotion and running adaptation of both herbivorous and carnivorous dinosaurs allowed them to roam large territories for food. The herbivorous dinosaurs were swift and agile and could easily escape from most predators. They exploited new resources for foraging. With long necks and tripodal pose (long hindlimbs and stout tail for support), they could reach high up in the foliage and pluck vegetation. They had specialized dentition for dealing with resistant plant material, the type that would be expected in drier climates.
In contrast, the herbivorous thecodonts such as aetosaurs foraged mainly on the ground. Their chance of survival deteriorated as the lowland plants became scarce in the more arid climates. With the emergence of the running prey population, thecodonts were unable to meet their food requirements. They could not escape direct interactions with the dinosaurs during the condition of extreme stress. They began to decline and soon became extinct at the end of the Triassic. Morphological novelties such as locomotory refinement, improved foraging techniques, and exploitation of new resources seem to have given dinosaurs a competitive edge over thecodonts.
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